1. Field of the Invention
This invention relates to a joint structure for vehicle body members comprising dissimilar metals in a vehicle such as an automobile.
2. Description of the Related Art
In recent years, vehicle body members made of aluminum or an aluminum alloy (for example, aluminum roof panel) have been utilized for achieving weight reduction of a vehicle body in a vehicle such as an automobile. In joining a vehicle body member made of aluminum and a vehicle body member made of steel, metallurgical joining (melt bonding such as spot welding) is not employed, since aluminum and steel are dissimilar metals, and their physical properties, such as melting point and coefficient of linear expansion, are different. Instead, mechanical joining using a fastener, such as rivet or bolt, or a caulking material is put to use (see Japanese Patent Application Laid-Open No. 2000-272541).
The above-described joint structure for dissimilar metals, however, has encountered the following problem: During joining, such as mechanical joining, or during baking in a painting step, distortion at a site along the junction occurs in the vehicle body member having lower rigidity, such as the aluminum roof panel, owing to a difference in stress or in a physical property such as coefficient of linear expansion.